In a broad sense, the goal of this project is to understand the regulation of the protein mass of skeletal muscle. The broader hypothesis being examined is that the regulation of the glucocorticoid sensitivity of skeletal muscle plays a major role in the regulation of the protein mass of skeletal muscle. The specific goal is to analyse changes in glucocorticoid sensitivity, the glucocorticoid receptor population and protein turnover following immobilization and following remobilization of an immobilized muscle. The experiments are designed such that they can be analysed from several different reference base perspectives as follows: 1) per muscle; 2) per muscle wet weight; 3) per muscle dry weight; 4) per mg DNA; 5) per mg cytosol protein; 6) per mg contractile protein; and 7) all of the above (i.e. 1-6) as experimental relative to contralateral control. The present proposal is based on a new hypothesis concerning the regulation of muscle mass. Substantial data are included which support the hypothesis. Because the loss of muscle mass plays a significant role in the progression of most dystrophies, the continued testing of this hypothesis is important. It should be apparent that if this hypothesis is not rejected, it will provide a new perspective with very substantial impact at the clinical level. The included experiments on immobilization/remobilization provide key information on which the hypothesis may be rejected or modified.